1. Field of the Invention
The present invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. Description of the Prior Art
As feature sizes in the production of integrated circuits approach 0.25 .mu.m and below, problems with interconnect RC delay, power consumption and crosstalk all become more significant. Integration of low dielectric constant (K) materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications partially mitigate these problems however, each of the material candidates having K significantly lower than dense silica suffer from disadvantages. A number of organic and inorganic polymers have K in the range of 2.2 to 3.5, however, these polymers suffer from problems including low thermal stability, poor mechanical properties including low glass transition temperature (Tg), sample outgassing, and long term reliability questions. One alternative is to employ nanoporous silicas which can have dielectric constants in the range of about 1 to 3. Nanoporous silica is attractive because it employs similar precursors (e.g., TEOS, tetraethoxysilane) as used for spin-on glasses (SOG's) and CVD SiO.sub.2 and because of the ability to control pore size of the nanoporous silica. In addition to low dielectric constant, nanoporous silica offers other advantages for microelectronics including thermal stability up to 900.degree. C., small pore size (&lt;&lt; microelectronics features), use of materials that are widely used in the semiconductor industry, namely silica and precursors (e.g., TEOS), the ability to tune dielectric constant over a wide range, and deposition using similar tools as employed for conventional SOG processing.
Nanoporous silica films can be fabricated by using a mixture of a solvent composition and a silica precursor which is deposited onto a wafer by conventional methods of spincoating, dip-coating, etc. The silica precursor is polymerized by chemical and/or thermal means until it forms a gel. Film thickness and density/dielectric constant can be controlled independently by using a mixture of two solvents with significantly different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The second solvent is then removed by increasing the temperature. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
Nanoporous silicas are preferably prepared from precursors comprising an alkoxysilane, a relatively high volatility solvent and a relatively low volatility solvent which is a polyol having an ether linkage. The principal reactions for an alkoxysilane such as tetraethoxysilane, tetramethoxysilane, triethoxysilane, methyltriethoxysilane, colloidal silica, and silica precursors which contain silicon-organic-silicon linkages are shown below. The following are exemplary reactions since the extent of hydrolysis and transesterification can vary from 0 to 4.
1) Hydrolysis: Si(OR).sub.4 +H.sub.2 O&lt;--&gt;Si(OR).sub.3 OH+ROH PA1 2) Transesterification: Si(OR).sub.4 +R"OH&lt;--&gt;Si(OR).sub.3 OR"+ROH PA1 3) Water condensation: Si(OR).sub.3 OH+Si(OR).sub.3 OH&lt;--&gt;Si(OR).sub.3 OSi(OR).sub.3 +H.sub.2 O PA1 4) Alcohol condensation: Si(OR).sub.3 OH+Si(OR).sub.4 &lt;--&gt;Si(OR).sub.3 OSi(OR).sub.3 +ROH PA1 5) Polyol transesterification: Si(OR).sub.4 +HOR'OH&lt;--&gt;Si(OR).sub.3 OR'OH+ROH PA1 6) Polyol condensation: Si(OR).sub.3 OR'OH+Si(OR).sub.3 OH&lt;--&gt;Si(OR).sub.3 OR'OSi(OR).sub.3 +H.sub.2 O PA1 a) blending a nanoporous silica precursor composition comprising at least one alkoxysilane; at least one relatively low volatility solvent composition comprising an ether of a C.sub.1 to C.sub.4 alkylene glycol which is miscible in water and alkoxysilanes, having a hydroxyl concentration of 0.021 mole/cm.sup.3 or less, a boiling point of about 175.degree. C. or more at atmospheric pressure and a weight average molecular weight of about 100 or more; at least one relatively high volatility solvent composition having a boiling point below that of the relatively low volatility solvent composition; optional water and an optional catalytic amount of an acid, thus forming a mixture and causing a partial hydrolysis and partial condensation of the alkoxysilane; PA1 b) depositing the composition onto a substrate while evaporating at least a portion of the relatively high volatility solvent composition; PA1 c) exposing the composition to a water vapor and a base vapor; and PA1 d) evaporating the relatively low volatility solvent composition, thereby forming a relatively high porosity, low dielectric constant, silicon containing polymer composition on the substrate. PA1 a) blending a nanoporous silica precursor composition comprising at least one alkoxysilane; at least one relatively low volatility solvent composition comprising an ether of a C.sub.1 to C.sub.4 alkylene glycol which is miscible in water and alkoxysilanes, having a hydroxyl concentration of 0.021 mole/cm.sup.3 or less, a boiling point of about 175.degree. C. or more at atmospheric pressure and a weight average molecular weight of about 100 or more; at least one relatively high volatility solvent composition having a boiling point below that of the relatively low volatility solvent composition; optional water and an optional catalytic amount of an acid, thus forming a mixture and causing a partial hydrolysis and partial condensation of the alkoxysilane; PA1 b) depositing the composition onto a semiconductor substrate while evaporating at least a portion of the relatively high volatility solvent composition; PA1 c) exposing the composition to a water vapor and a base vapor; and PA1 d) evaporating the relatively low volatility solvent composition, thereby forming a relatively high porosity, low dielectric constant, silicon containing polymer composition on the substrate.
Usually, an alcohol is used as the solvent which is the same as the alkoxy group on the silane. When polyols such as ethylene glycol or glycerol are employed, the situation is more complex. For a typical polyol, HOR'OH, transesterification can occur as:
The extent of transesterification depends upon the relative concentrations of water, alcohol and polyol as well as the reactivity of both the alcohol and polyol. The product of the right side of equation 5 can undergo additional water or alcohol reactions which yield a SiOR'OSi linkage as shown in equation 6.
Without the polyol, polymerization can occur only through SiOSi linkages but with the polyols, additional reaction pathways are created. The extent of modification that the polyol will have on the nanoporous silica evolution depends upon a number of factors which cannot be anticipated. It has now been found that the low volatility polyol solvent must meet a number of criteria which were previously not known in order to achieve a stable precursor solution. In addition to having a high boiling point and proper solubility in water and alkoxysilanes, important criteria are low hydroxyl concentration and high molecular weight.
An important criteria for obtaining polyol-based precursor solutions which are stable over long time (i.e. months) and which will give nanoporous silica films with desirable properties such as high surface area, high mechanical strength, and small pore size, is that solvents be employed with volumetric hydroxyl concentrations in the correct range. If the concentration is too high, reactions 5 and 6 will proceed to excess and the precursor molecular weight will grow with time before deposition. If the concentration is too low, the film will not exhibit desirable properties. The desired range of hydroxyl concentration depends on the target dielectric constant. The vapor pressure, and hence drying rate, of a fluid within a pore is different than for the pure fluid. This reduction in vapor pressure is a result of curvature of the liquid meniscus at the vapor-liquid interface. As the molecular weight increases, the reduction in vapor pressure increases and the drying rate decreases.
It has now been found that one group of low volatility solvents that is uniquely suited to meeting each of these constraints are the ethers of ethylene glycol and propylene glycol. The compounds useful for this invention have ether linkages (C--Q--C) and are polyols (multiple alcohol groups on the same molecule). A single compound may be used or a mixture of compounds may be employed to achieve the desired properties.